Multiwavelength lasers (MWLs) have great potential in a variety of civilian and military applications, enabling the increased transmission rates of wavelength-division multiplexing (WDM) systems, and enhanced operation in free-space settings such as range-finding and beam guidance. Ideally, MWLs should have low inter-channel interference (crosstalk), high power, low beam divergence for optimum coupling or free-space propagation, and be compact. In addition, it is highly desirable that any associated tuning circuitry be as simple as possible for ease of packaging and control, and that device size and complexity scale well as the number of wavelengths increases. No existing MWL design achieves all these ideals. This is due in part to gaps in the understanding of gain cavity behaviour under multiwavelength lasing conditions, but also to limitations inherent in established laser designs.
A number of MWL schemes have been developed, and generally fall in two categories: array and shared-gain. Array MWLs consist of a row of single-wavelength laser (SWL) designs, along which some wavelength-selecting parameter is varied. They offer the advantages of being a relatively simple extension of SWLs, and allow straightforward individual modulation of each laser. However, such devices are prone to cross-talk from independent drifting of individual wavelengths; subject to channel deviations from fabrication imprecision; and suffer from low yield. For shared-gain MWLs, channels share a gain region integrated with multi-resonance feedback elements, yielding a wavelength comb whose spacing is maintained even in the event of overall drifting. However, gain-coupling cross-talk must be properly treated when the wavelength spacings are too small (&lt;1 nm), and individual channel modulation can be more difficult. The performance characteristics of current MWL designs are summarized below in Table 1.
As can be seen, impressive individual characteristics have been achieved. However, no single device combines the virtues of high power, large channel density, and low divergence. In fact, all designs suffer from high divergence: near-field beam size is no more than a few .mu.m, which (for .lambda..about.1 .mu.m) corresponds to a divergence of at least .about.10.degree.. The ideals of high power and low divergence are in contradiction due to the requirement of monomode operation, which for existing MWLs restricts both current density and beam width.
The ideals of low divergence and high-power have been realized concurrently at a single wavelength in a ring laser configuration as disclosed in V. A. Sychugov, A. V. Tishchenko, A. A. Khakimov, "Nonlocalized Bragg Mirror Of The Comer-Reflector Type", Soviet Technical Physics Letters, 5 1270-1274, 1979., and refined by K. M. Dzurko et al, see K. M. Dzurko, D. R. Scifres, A. Hardy, D. F. Welch, R. G. Waarts, and S. O'Brien, "500 mW coherent large aperture ring oscillators", Electronics Letters, 28 1477-1478, 1992. In both implementations, conventional single-pitch gratings were empilayed, and output was single-wavelength only.
To overcome the aforementioned shortcomings, there is a need for a lasers which simultaneously permit broad-beam collimation and monomode operation, with simultaneous emission of multiple wavelengths